The present invention relates to the orientation of antennae on a satellite. It relates to antenna composed of a horn member fixed to the body of the satellite and mounted generally on a tower and a movable reflector, mounted on the body of the satellite, through an orientable platform, for example, as the orientable platform apparatus described by Hubert et al. in their U.S. patent application Ser. No. 125,058 filed on Feb. 27, 1980. This is now U.S. Pat. No. 4,325,586 issued Apr. 20, 1982.
To orient such an antenna with good accuracy, recourse is generally had to a radio-frequency sensor which delivers an error signal proportional to the sighting deviation relative to an earth beacon. This signal can be used to control the orientable platform as, for example, explained in the aforesaid U.S. patent application. The considerable advantage of the radio-frequency sensor is to use a beam which follows an electromagnetic path as close as possible to that which is followed by the beam of the mission. In particular, the antenna being a deformable assembly, the horn plus the reflector, it is essential for the sensor to use the reflection on a surface connected to the reflector. A drawback of the radio-frequency sensor is its complexity which is manifested by a rather low reliability. One solution could be to double or even to triple the corresponding electronic systems but an increase in weight would result therefrom.
Infra-red earth sensors, usually two in number for reasons of reliability on geostationary satellites, sight the center of the earth seen from the satellite. They are used for the acquisition of the earth and for the stabilization of the satellite in geosynchronous orbit.
The body of the satellite with one or two or several inertia wheels or reaction wheels according to conventional arrangements may be stabilized toward the center of the earth by means of signals from the earth sensor and the movable antenna can be mounted on this satellite through an orientable platform controlled, independently, toward any point on the earth constituted by a beacon, by means of signals from the radio-frequency sensor.
When the satellite is redirected with respect to the earth by an angle .theta. along one axis, the platform of the antenna pivots substantially by .theta./2 with respect to the satellite to ensure the sighting of the beam cancelling the error signal of the radio-frequency sensor.
It is an object of the present invention to control the body of the satellite itself to limit the deflections of the platform and to use, as an error signal to control the body of the satellite, any signal characterizing the pivoting of the platform about two axes.
The control current of the pivoting of the orientable platform which may be substantially proportional to the pivoting angle of the platform, as explained in the aforesaid U.S. patent application, is utilized to this end.
The platform, used to detect the position of the satellite in space, is hence in accordance with the invention, considered as a particularly simple and reliable sensor.
More precisely, according to the present invention a method is provided to control the orientation of an antenna on a satellite possessing at least one antenna comprising a horn rigidly connected to the body of the satellite, a movable reflector mounted on an orientable platform and at least one radio-frequency sensor using this antenna to control the orientation of said reflector, characterized in that is consists of:
using a signal characterizing the pivoting of the orientable platform with respect to the body of the satellite to orient said satellite correctly with respect to the reflector, in order to limit the pivoting of said orientable platform, and
using the signals from at least one earth sensor in the same way as the signals of the radio-frequency sensor to control said reflector, so as to orient the beam toward the point sighted on the earth.
According to another aspect of the invention, a configuration of sensors is provided on a satellite, applying the above method and enabling the use of the infra-red earth detector in redundancy to the radio-frequency sensor to control the antenna whilst the body of the satellite is controlled only with the position signal of the platform, itself orientable with respect to the satellite.
Other characteristics and advantages of the present invention will emerge from the description which follows given with reference to the accompanying drawings.